Take the natural expansion and contraction of wood into account when making joints

by Bob Duncan

This article was first published in issue 53 of Scroll Saw Woodworking & Crafts.

Wood is porous, so it absorbs moisture from the air around it. As the amount of moisture in the air changes, depending on the season and the climate control where the wood is stored, the wood expands and contracts. It can move enough to warp a board or break apart joints, and the larger the piece of wood, the more it will move. Plus, different types of wood move at different rates. So, when you plan to make a project from solid wood, you need to take wood movement into account or your beautiful frame or stunning box can literally break apart.

To avoid problems with wood movement, use manufactured wood, such as plywood, particleboard, or MDF. Because they are made of ground-up wood fibers or thin sheets of wood glued together with alternating grain directions, these types of wood are basically as stable as a piece of plastic. However, they aren’t as attractive as solid wood.

End-to-end butt joint

Miter joint

A better way to avoid problems is to use strong joints. A carefully chosen and well-built joint accounts for wood movement and can be stronger than the wood around it. Let’s examine a few types of common joints and the ways you can use them to build strong projects. For more on wood glue and how to use it, see page 31.

Panel joint

About Grain

Trees are made of hollow vessels that transport water and food up and down the trunk. These vessels give cut wood the visible lines we call grain. Think of a piece of wood like a bundle of straws—the long sides of the straws are the faces and edges of the wood, and the open ends of the straws are the cut ends, or end grain, of the wood. Wood joints are strong when faces and edges are joined, but weak when the ends are joined because the glue gets absorbed into the open “straws.”

Edge joint

Avoid

End Grain Joints

Joints made by gluing end grain are weak and often break apart if you rely only on the glue, which absorbs into the wood. If you must glue end grain, reinforce the resulting butt joint with brads, dowels, a corner block, or metal corner braces.

Miter Joints

A miter joint consists of two ends cut at angles and glued together. While they look nice from the outside, miter joints are extremely weak glue joints because they essentially create a double end-grain joint. If you really want to use a miter joint, reinforce it with small brads, a corner block, or a metal corner brace.

Dade joint

Use

Panel Joints and Edge Joints

When you glue the edges of two pieces of wood together with the grain running the same direction, it’s called a panel joint. These joints are extremely strong as long as the edges are flat, square, and smooth. An edge joint is similar, but instead of joining the two edges, you join the edge with the flat side of another board. This is also a strong joint.

Finger joint

Grooves, Dados, and Rabbets

Grooves, dados, and rabbets are all types of notches in the wood, and they are often used to add a bottom to a box. If a box bottom is made with solid wood and glued in place, it will expand and break the box apart. To avoid the problem, cut grooves in the box walls that are slightly wider and deeper than the box bottom is thick and long. Assemble the box walls and “float” the bottom in the grooves without gluing it, thus giving the bottom panel space to expand as needed.

Dovetail joint

Box joint

Dovetails, Finger Joints, Box Joints

These joints all use interlocking pins to assemble pieces both by fitting together and by offering a much greater glue surface than that of a flat piece. You will often see these strong joints used for boxes and drawers; dovetails, especially, will hold together even without glue. While it is simple to use routers and templates to cut these joints, the templates limit the size and shapes of the pins. Many woodworkers use handsaws and chisels to create custom pin sizes.